Employee Retirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Amounts recognized in the consolidated balance sheets consisted
Retiree health benefit obligation	$ 6,767		$ 6,656
Pension plan
Components of net periodic cost:
Service cost	80	$ 64
Interest cost	410	372
Expected return on plan assets	(456)	(407)
Amortization of net (gain) loss	181	255
Net periodic cost	215	284
Employer contributions during the period	193	226
Pension plan | Minimum
Components of net periodic cost:
Estimated total required contributions	967
Other postretirement benefit cost
Components of net periodic cost:
Service cost	53	57
Interest cost	70	64
Amortization of net (gain) loss	(32)	(17)
Net periodic cost	$ 91	$ 104